A particle concentration fluorescence immunoassay for the quantitative determination of human alpha 1-antitrypsin in Escherichia coli is described. The principle advantages of this system are speed, automation and a high level of sensitivity compared with enzyme assays and Western blot analysis. Two monoclonal antibodies recognizing different epitopes on the protein are used for the detection and quantitation of alpha 1-antitrypsin. This method has a measured range of 30-2500 ng/ml with a mean accuracy of 5.5% and a precision of 5.1%.